1. Field of the Invention
The present invention relates to the electrode structure of a spin transistor and a method of manufacturing the same.
2. Description of the Related Art
Recently, a magnetic memory (to be also referred to as an MRAM (Magnetic Random Access Memory) hereinafter) using an MTJ element having a magnetic tunnel junction (to be also referred to as an MTJ (Magnetic Tunnel Junction) hereinafter) made of a stacked structure including a ferromagnetic material layer/insulator layer (tunnel barrier)/ferromagnetic material layer as a memory element has been proposed. In this MRAM, the resistance of the stacked structure is changed by magnetization direction in one ferromagnetic material layer (a reference layer or fixed layer), and controlling spins in the other ferromagnetic material layer (a recording layer or free layer), thereby storing a high-resistance state or low-resistance state by making it correspond to data “0” or “1”. For example, the resistance is low when the spins in the reference layer and recording layer are parallel, and high when the spins in these layers are antiparallel. The magnetoresistance ratio (MR ratio) of this MTJ element has recently reached 500%, although it was a few 10% at room temperature a few years ago. This expands the possibilities of MTJ elements, enabling a variety of spin devices as well as the MRAM to be obtained. As an example, a spin MOS field-effect transistor (to be referred to as a spin MOSFET hereinafter) has been proposed (see, e.g., S. Sugahara and M. Tanaka, Appl. Phys. Lett. 84 (2004) 2307). This spin MOSFET uses a ferromagnetic material in the source and drain electrode portions of a normal MOSFET. This employs the degree of freedom of spins of carriers in the device operation. By using this function, the MTJ element is expected to be applied to a reconfigurable circuit such as an FPGA.
As described above, the MR ratio of the MTJ has increased. However, to control a spin-polarized electric current by the gate voltage in the spin MOSFET, it is important to inject an electric current (to be also referred to as a highly spin-polarized electric current hereinafter) having a highly spin-polarized electron ratio from a magnetic layer in a source portion into a channel. Also, in the spin MOSFET and MTJ, the magnetoresistive effect (MR) controlled by the relative magnetization directions in two magnetic layers sandwiching a non-magnetic layer is the basic operation principle of the device.
According to the basic principle of the spin MOSFET, it is essential to achieve an appropriate interface resistance between a semiconductor substrate and magnetic layer. As a method of controlling the interface resistance, a method of inserting a tunnel barrier layer has been disclosed. On the other hand, a method of forming a heavily doped impurity region in a semiconductor substrate by ion implantation has been used as a method of achieving a low resistance. To improve the performance of the spin MOSFET, it is necessary to suppress the interface resistance to typically about 10 Ωμm2. Also, when performing spin-torque transfer (STT) in a write method using spin transfer in the spin MOSFET and MTJ, no spin reversal occurs unless an electric current having a very high current density is supplied to an element. When an electric current having a high current density is supplied to a magnetoresistive effect element having a tunnel barrier layer, element destruction occurs because a high electric field is applied to the tunnel barrier. Accordingly, a structure that causes spin reversal by an electric current having a low current density is required for STT. This also makes it essential to decrease the interface resistance.
As the electrode structure of the spin MOSFET, (1) a Schottky barrier type electrode structure and (2) a tunnel barrier type electrode structure have been proposed. As electrode structure (1), a low Schottky barrier of MnAs/Si has been reported (see, e.g., K. Sugiura et al., APL 89 (2006) 072110).
As explained above, in the magnetoresistive effect element, magnetic memory, and spin MOSFET, it is essential not only to generate and inject a highly spin-polarized electric current but also to control the interface resistance in order to improve the performance. In the interface of a metal layer/Si, the resistance increases because a Schottky barrier is formed. Also, even in a metal layer/tunnel barrier/Si, the resistance increases because a depletion layer behaved as Schottky-like component is formed in the tunnel barrier/Si by the influence of Fermi level pinning. Conventionally, these Schottky components are effectively reduced by forming a heavily doped impurity region in a semiconductor substrate by ion implantation. On the other hand, a simpler process is necessary for applications. Therefore, demands have arisen for a technique that decreases the resistance by new impurity engineering in the interface between a magnetic material and Si.
One promising solution for improving the performance of the spin MOSFET is an electrode structure obtained by combining a Heusler alloy and crystalline tunnel barrier. However, a general conventional method is to form a Heusler alloy by sputtering by using a sputtering target having a matched composition ratio. When directly depositing a film on an Si substrate, therefore, impurity diffusion from the Si substrate has an influence on the crystallinity. In addition, annealing is performed to improve the crystallinity after the Heusler alloy has grown. Since this mixes Si and the magnetic material around the interface, the magnetic properties degrade.